The following invention relates to a thin-film electroluminescent (TFEL) device for providing an improved optical display. More particularly, the invention relates to a thin-film absorption layer within the device for absorbing incident light.
TFEL displays are constructed of a laminar stack comprising a set of transparent front electrodes which are typically made of indium tin oxide formed on a transparent substrate (glass), and a transparent electroluminescent phosphor layer sandwiched between transparent dielectric layers situated behind the front electrodes. Disposed behind the rear dielectric layer are rear electrodes which are usually constructed of aluminum because it provides both good electrical conductivity and a self-healing failure characteristic. Aluminum rear electrodes also enhance the luminescence of the display by reflecting back towards the viewer most of the light that would otherwise be lost to the rear of the display. This reflected light nearly doubles the light of the displayed image because the phosphor layer emits light that is directed in equal amounts in both the forward and rearward directions. However, the aluminum rear electrodes also reflect forward ambient light entering from outside of the display which is superimposed with the display information thus reducing its contrast.
To increase the contrast of the display, an antireflection coating is sometimes used on the front transparent substrate of the display to reduce the amount of ambient light reflected from the front of the display. The TFEL laminar stack may further include an enclosure seal against the substrate, with the rear wall of the enclosure blackened to block light entering from extraneous light sources behind the display. The black coating absorbs ambient light passing through the display from the front that was not reflected by the rear electrodes.
The reflection off the rear electrodes, which are typically aluminum, has a diffuse reflectance due to the surface roughness of the reflective rear electrodes, which in turn adds to the diffuse scattering from other thin-film layers of the display. The diffuse reflectance is typically measured with a photometer placed in the viewing position and perpendicular to the display. With ambient light directed at the display from a 45 degree angle to the perpendicular viewing direction, a typical TFEL display has approximately 15% diffuse reflectance. A circular polarizer filter reduces the diffuse reflectance from about 15% to about 1%, but transmits only about 37 to 42 percent of the emitted light from the display and adds substantially to the cost of the display.
Another approach that has been tried for improving the display contrast is to use indium tin oxide transparent rear electrodes. This reduces the reflectance of light off the rear electrodes and permits light to pass on through to the back of the display where it may be absorbed. However, indium tin oxide is of higher resistivity than metallic electrodes, such as those made of aluminum, and therefore must be made much thicker to achieve adequate electric conductivity. Further, thick layers of indium tin oxide do not exhibit the self-healing characteristics of aluminum rear electrodes. This can lead to an unacceptable loss in device reliability due to dielectric breakdown.
In yet another approach, shown in Steel et al., U.S. Pat. No. 3,560,784, a light-absorbing layer is incorporated into the thin-film laminate structure. This reference suggests that if a conventional metallic rear electrode is used, then a light absorbing layer may be added as an insulating layer or as a conductive layer. Insertion of a dark layer immediately behind the phosphor layer, however, can interfere with the phosphor/insulator interface leading to inferior display performance. The light pulse for one polarity may be reduced which can give rise to a flicker effect as well as to a loss in overall brightness.
Shimizu, U.S. Pat. No. 5,003,221 discloses the use of a thin-film layer that is formed between a transparent substrate and a layer formed adjacent to the transparent substrate in a TFEL laminar stack. The refractive index of the thin-film layer is made to change to approximate that of layers toward the interfaces between the thin-film layer and the corresponding layers, so that a difference in the refractive index at these interfaces is minimized. Shimizu is directed to solving the problem of maximizing the transmission of light between layers by using different real indexes of refraction. To this end, Shimizu teaches the use of multiple graded layers or a single continuous gradation, of the thin-film layer between two adjacent layers of the laminar stack to maximize the transmission of light between those respective layers.
Upon the application of an electric field between the transparent electrode layer and the rear electrode layer, light-emitting pixels are formed in the phosphor layer. Due to the physical structure of the phosphor layer, the pixels emit light mostly directed within the plane of the phosphor layer. As the emitted light travels in the phosphor layer, it is scattered by defects in the phosphor layer causing a substantial portion of the emitted light to be directed in the forward and rearward directions. Light that is directed or scattered rearwardly will be reflected forward off the rear electrodes adding to the forwardly directed light. This causes a fuzzy-looking region to appear around the addressed pixel. A circular polarizer cannot selectively reduce this effect without decreasing the overall amount of emitted light.
Emitted light striking the phosphor-dielectric boundaries at small angles to the boundaries is totally internally reflected. Light striking the phosphor-dielectric boundaries at a greater angle of incidence, refracts into the respective dielectric. The difference between the indexes of refraction of the front glass and exterior air are substantial, so the required angle of incidence for light to refract into the air is greater than all other internal interfaces. In other words, light that cannot escape the front glass-air boundary may freely refract between other layers of the TFEL laminar stack. The rearwardly directed light reflected off the glass-air boundary, and other rearwardly directed light, will impact the rear electrodes causing the light to reflect forward. Such light is either diffusely scattered or strikes a defect, thereby, will travel within the display until it possibly increases its angle with respect to the glass-air boundary permitting its escape from the display generally at a location distant from the pixel.
The apparent color of a TFEL display appears to depend upon the wavelengths of light emitted by the phosphor layer and the thicknesses of the individual thin-film layers. Amber displays typically vary in color from yellow at a perpendicular viewing angle to red/orange when viewing off the perpendicular viewing angle. Alternatively, the color of the perpendicular viewing angle may be red/orange and when viewing off the perpendicular viewing angle the color may be yellow. With current processing techniques for depositing the thin-film layers, it is hard to control the process sufficiently to produce consistent colors from screen to screen.
What is desired, therefore, is a way to enhance the contrast of the display, reduce the fuzzy looking region around the addressed pixel, reduce color variations between displays, and nearly eliminate the diffuse reflectance.